Various resource deposits can be mined from the Earth using man and machine entry techniques. With respect to resource deposits that are soluble, solution mining techniques can be used to remove the resource to the surface. In particular, solution mining involves dissolving a target evaporite in a solvent in situ to form a pregnant brine, and removing the pregnant brine to the surface. Evaporation, for example solar evaporation or evaporation aided by the addition of heat from a fossil fuel source, is then used to separate the target resource from the solvent.
Accordingly, solution mining requires transforming the target resource, such as halite (rock salt) or sylvite (potash), from a solid crystalline form to a brine. In particular, these salts are target minerals that will dissolve when wetted by the solvent to form the brine. The brine, replacing the volume of the target mineral in the crystalline state, is pumped from its below ground location to the surface and eventually to evaporation ponds or facilities. The rate of change from crystalline form to a dissolved form is a function of solvent temperature, purity (lack of solutes), agitation, and fluid pressure. As the goal is to produce a saturated brine (also known as a pregnant brine), purity cannot be positively affected, except that a solvent that is uncontaminated by other solutes can be applied. Agitation and control of solute temperature are variables that may be controlled to enhance productivity. Productivity of a bore may be defined as the total rate of change, measured in tons per day, of transformation of the target mineral from a crystalline state to a brine within the affected area of the bore.
In one approach, the resource deposit is accessed using a vertical access shaft. Because many resource deposits that are the target of solution mining are in the form of horizontally planar deposits, a vertical well typically provides a very limited area over which the bore perpendicular to the plane of the deposit in contact with the mineral resource. This limited surface area means that the area of the resource deposit exposed to the solvent is severely limited. This in turn limits the amount of the resource that can be placed in solution per unit time.
In order to increase the surface area of the resource deposit that can be contacted with solvent, horizontal bores can be formed using directional drilling techniques. In particular, bores can be formed that run through the resource deposit according to such techniques. Moreover, multiple horizontal bores in various patterns, such as an X, a fan, or rectangular grid, can be used. However, because the initial area of exposed resource deposit is limited to the area of the one or more bores within the resource deposit, the amount of the resource that can be placed in solution per unit time remains limited.
Further, such mineral formations may be vertically thin and of great horizontal size. The process of deposition is the evaporation of ancient inland seas that occurred when a saltwater inlet became cut off geographically from the main sea. The shallow areas experienced oversaturation of the brine as the water level dropped due to evaporation resulting in deposition of salts as various salinity and atmospheric conditions were reached. As the most valuable mineral tends to be a small fraction of the dissolved solids found in sea water, the thickness of the deposited layer is typically thin. Therefore a way to capitalize on relatively common vertically thin and horizontally broad deposits will make otherwise valueless deposits of economic interest.
At least partially as a result of these limitations, major mining operations of resource deposits, such as potash, typically utilize man and machine entry techniques and occur only in areas with exceptionally large deposits. Large not only in breadth, but also in depth. For this rare situation to occur, the amount of evaporation need have been extreme with conditions remaining stable for decades or longer. Such a deposit did occur in central Canada around the present day location of the city of Saskatoon. The deposit is deep (3000 plus feet below the surface) but incredibly rich. The richness of the deposit is indicated by the fact that this region currently produces about ⅓ of the global potash usage. However, because this deposit is in a climactic area that is not amenable to the use of solar evaporation to recover the resource from the pregnant brine, heating, for example by burning natural gas, is required. Therefore, large amounts of energy must be expended in connection with such mining operations. Conversely, areas with large amounts of resource deposits that are in relatively thin, planar formations, do occur in locales in which solar evaporation could be used efficiently. However, conventional mining of such deposits has typically been uneconomical. Vertically deep deposits lend themselves well to man entry techniques, vertically thin deposits require novel and inventive means to claim them from the Earth.